1. Field of the Invention
The present invention relates to a method for the function control of a position measuring system with a scanning unit, which can be displaced with respect to a graduation, wherein the scanning unit has several detector elements and generates several scanning signals at an instantaneous relative position between the graduation and the scanning unit. The present invention also relates to a position measuring system for executing the method.
2. Discussion of Related Art
Position measuring systems in the form of angle and linear measuring devices are widely employed in the machine tool industry and in other manufacturing, manipulation and testing systems. A high degree of functional dependability increases in importance in all types of usage, since faulty operations can cause considerable damage. There have already been several suggestions for a solution by obtaining a timely error indication through monitoring tests and in this way to prevent resulting damage.
A method, as well as a device for monitoring the correctness of the detected measured position value is described in U.S. Pat. No. 3,725,904, the entire contents of which are incorporated herein by reference, DE 28 25 842 C2 and corresponding U.S. Pat. No. 4,311,987, the entire contents of which are incorporated herein by reference. Two scanning units, which are arranged offset from each other by a predetermined distance, are provided for scanning a coding, or a graduation. The two measured position values of the two scanning units are compared to each other and a position difference is formed. This measured difference is compared with a nominal difference, predetermined by the geometric arrangement of the scanning units, and an error signal is issued as a function of the result of the comparison.
This step has the disadvantage that a redundant scanning unit is required, which increases the structural size and the output for structural elements.
Only one scanning unit is required with the position measuring device in accordance with DE 40 09 749 A1. For performing the test procedure, the scanning unit is displaced over a distance fixed by stops, and the distance measured in this way is compared with the distance defined by the stops. An error signal is generated when there is a deviation.
The disadvantage of this step lies in the provision of mechanical stops and the provision of an additional drive mechanism, which considerably increase the structural size.